1. Field of the Invention
This invention relates to a filter used mainly in microwave and millimeter bands, which is constructed using a coupled transmission line system including a capacitive coupling section.
2. Prior Art
The prior art coupled transmission line system 10 including a capacitive coupling sections 11 at the input and output ends in a filter comprising series arranged half wavelength (λ/2) or quarter wavelength (λ/4) resonators utilizing a conventional coplanar line is described taking the coupling section 11-1 at the input end of the filter as shown in FIG. 1 as an example. Such coupled transmission line system 10 comprises a pair of ground conductors 13 and a line conductor 14 formed on a dielectric substrate 12, the line conductor 14 being disposed between the ground conductors 13 and including a line conductor section 14-1 on the input port side and an opposedly facing line conductor section 14-2 of a first resonator having a certain characteristic impedance, the opposed ends of the two line conductor sections being separated by a meander-like inter-digital gap. It has heretofore been a common practice to use a structure having a meander-like gap with very small gap widths g1, g2 as compared to the gap width g0 at the capacitive coupling section 9-2 (see FIG. 2) between the resonators.
Examples of the filter utilizing such construction include the λ/4 resonator coplanar line filter as disclosed in a non-patent literature 1-A: H. Suzuki, Z. Ma, Y Kobayashi, K. Satoh, S. Narahashi and T. Nojima, “A low-loss 5 GHz bandpass filter using HTS quarter-wavelength coplanar waveguide resonators,” IEICE Trans. Elect., Vol. E85-C, No. 3, pp. 714-719, March 2002 and a non-patent literature 1-B: Suzuki, Ma, Kobayashi, Satoh, Narahashi and Nojima, “Design of 5 GHz 10-pole Bandpass Filters Using Quarter-Wavelength Coplanar Waveguide Resonators,” Technical Report of IEICE, SCE2002-9, MW2002-9, pp. 45-50, April 2002 and the compact inter-digital bandpass filter using coplanar quarter-wavelength resonators as disclosed in a non-patent literate 2: Ma, Nomiyama, Kawaguchi and Kobayashi, “Design of Compact Inter-digital Bandpass Filter Using Coplanar Quarter-Wavelength Resonators,” Technical Report of IEICE, SCE2003-12, MW2003-12, pp. 67-72, April 2002.
The four-stage λ/4 resonator coplanar line filter 8 disclosed in the non-patent literature 1-A and 1-B is shown in FIG. 2 in which the reference numeral 11-1 indicates a conventional capacitive coupling section as shown in FIG. 1 which is used at the input end of the filter. Indicated by 9-6, 9-7, 9-8 and 9-9 are four stage resonators, the first and second resonators and the third and fourth resonators being coupled by inductive coupling sections 9-3 and 9-4, respectively while the second and third resonators are coupled by a capacitive coupling section 9-2. The fourth resonator and a line conductor section 14-4 on the output port side are coupled by a conventional capacitive coupling section 11-2 as shown in FIG. 1 as is the case with the input end. It is to be noted that in FIG. 2 the parts that are similar to like parts in FIG. 1 are indicated by like reference numerals. Further, the capacitive coupling section 9-2 for coupling the second and third resonators will be referred to as capacitive resonator coupling section herein-below in order to discriminate it from the capacitive coupling sections 11-1, 11-2 for the input and output ends.